The present invention relates to lasers and more particularly to electric discharge gas lasers (EDL) wherein an electric discharge in a gaseous medium produces a population inversion of energy states in the medium sufficient to support the laser action.
A laser beam is a beam of coherent electromagnetic radiation which by virtue of its coherence is highly directional and so the laser beam can be projected great distances with little spreading of the beam. Because the laser beam possesses space coherence, it can be focused to form a small spot. Hence, enormous power densities can be obtained.
An electron beam laser is described in U.S. Pat. No. 3,702,973, issued Nov. 14, 1972 entitled "Laser or Ozone Generator in Which a Broad Electron Beam with a Sustainer Field Produce a Large Area Uniform Discharge" to Daugherty et al. This patent describes a structure and method for operating a relatively large diameter, high pressure gas electric discharge gas dynamic laser in which the medium contains CO.sub.2. A spacially uniform controlled electric discharge is produced in the working region by introducing ionizing radiation (a broad electron beam) into the laser optical cavity through a wall of the cavity to produce a substantially uniform predetermined density of secondary electrons in the gaseous medium by ionizing the medium and at the same time providing a sustainer electric field which is uniform throughout the working region of the laser and which provides a predetermined electron temperature which is calculated to increase the average energy of secondary electrons in the working region without substantially increasing the predetermined electron density in the region. This patent describes a method and structure for producing a uniform controlled discharge in a gaseous medium in a relatively large tube at relatively high pressure. The sustainer field direction, the laser beam direction and the gas flow direction may be mutually orthogonal.
In operation, the ionizing electron beam is generated outside the laser cavity by an E-beam generator and there is a broad area uniform beam of sufficient cross section dimension to cover the relatively large working region of the laser. A suitable structure for generating such a broad area uniform electron beam is described in U.S. Pat. No. 3,749,967 which issued July 31, 1973 entitled "Electron Beam Discharge Device" by Douglas-Hamilton et al. The beam is transmitted into the laser cavity through an electron window and into the working region bounded by the sustainer field. A portion of the laser optical cavity is included in that sustainer field and in the optical cavity.
In the high power electron discharge such as described in the above-mentioned U.S. Pat. No. 3,702,973, the output laser power is approximately proportional to the input power to the sustainer section. The sustainer section includes an anode and a cathode with the gaseous working region in between and so the working region of the laser is defined by this anode and cathode. It is the discharge between the anode and the cathode, uniformly maintained, that pumps the laser and so provides the inverted population of energy states necessary for laser action. Since the laser output power is proportional to the input power to the sustainer, the output power can be controlled by controlling the sustainer voltage. This technique has been effective for gas lasers of smaller size. However, it is not as effective for lasers of larger size, particularly where the laser output power must be changed rapidly. For relatively large electron beam lasers, the density of the electron beam projected into the working region between the sustainer electrodes is controlled while the sustainer voltage is held constant. Thus, the sustainer current is varied to vary output power of the laser. This, in turn, depends upon the ion concentration produced in the working region by the ionizing electron beam.
The ionizing electron beam is produced by the E-beam system which is an external electron accelerating device that generates a broad area electron beam which is projected through an electron window into the working region of the laser. In the E-beam device, electrons emitted by a cathode are accelerated by anodes and so the energy of the electron entering the working region of the laser is determined by the accelerating anode voltage. Usually, the accelerating anode voltage is maintained constant and the voltage on a control grid located between the accelerating anodes and the cathode is varied. This grid controls the density of the electron beam that is injected into the sustainer working region of the laser. Very abrupt changes in the laser output power can be achieved by abruptly changing the voltage on this control grid in the E-beam system. Thus, the E-beam device and the sustainer device operate in conjunction in a fashion similar to a tetrode vacuum tube to control the output power of the laser, that output power being controlled by a grid potential in the E-beam device.
Heretofore, an electron beam CO.sub.2 laser constructed and operated as described above and including an E-beam device and a sustainer has included a null-type feedback control system. The feedback control system detects or senses the current in the electron beam and compares that current with a standard preset by the operator, producing a control signal that represents the difference. The control signal is applied to the E-beam device control grid. That feedback system, intended particularly to compensate for variations which might result from such things as changes in power line voltage, drifts in component values in the power supply or other factors that could affect the amplitude of the E-beam current. It was not completely effective to correct perturbations in the laser beam output and so was less effective than desirable where the output laser beam must be maintained steady and substantially free of perturbations or where the beam power must be changed abruptly as when the beam is pulsed.